Unwanted deposits can occur in many industrial systems. For example, organic and silica/silicate deposits are a problem in some boilers and evaporators used in thermal recovery systems. The presence of deposits can significantly reduce system thermal efficiency and productivity, increase operating/maintenance costs, and in some cases lead to equipment failure. Steam generators and evaporators are especially prone to deposits due to operation at elevated temperatures, pH and increased cycles of concentration (COC).
In particular, deposits are prevalent in produced water (steam assisted gravity drainage (SAGD), steam flood, etc.) plant unit operations. For example, SAGD operations inject steam into geological formations to stimulate the production of bitumen or heavy hydrocarbon. Oil Sands deposits in Alberta, Canada represent an area where this process is extensively used. Pairs of horizontal wells are bored into the oil-containing formation. The upper well injects steam and the lower well which is positioned below the steam injection line, continuously extracts a complex emulsion. That emulsion contains bitumen and water. The emulsion is broken; the bitumen is sent for refining, while the produced water (separated from the emulsion) is treated and reused as feedwater for the steam generators.
This SAGD process for producing bitumen results in large volumes of organic-laden and silica-laden water. There are two options typically used for treating the returned produced water and supplemental makeup water for use as feedwater for steam generation. The first option is warm lime softening (WLS) and is the more traditional method for treating produced water. The treated water quality is poor relative to ABMA/ASME boiler feedwater standard guidelines. However, the use of once-through steam generators (OTSG) mitigates the need for high purity water. In a preferred operation mode of the OTSG, the feedwater can have less than 8000 mg/L total dissolved solids (TDS) and near zero total hardness and the silica (SiO2) specification is typically less than 50 mg/L. The WLS/Ion exchange process can achieve these requirements.
Evaporation technology (in particular mechanical vapor compression (MVC)) is the second and newer option of water treatment. The main reason for using evaporators to treat produced water is to achieve a very high quality of water so a conventional drum boiler can be used instead of OTSG. However, in some cases, evaporators are used to clean extremely dirty produced water along with other waste streams and other water sources for use as feedwater in OTSG. As the industry looks to more and more recycled water, evaporators will play an important role in treating waste water for reuse. This can be accomplished because the evaporation technology is very robust and can be used on the more difficult to treat waste waters.
With evaporators, a high percentage of produced water is recovered as high quality boiler feedwater. High quality feedwater produced from evaporation enhances reliability of the steam generation equipment. The evaporator footprint is also significantly smaller than conventional WLS treatment.
Because of the nature of the water being treated, evaporators are likewise subject to deposition. Chemical treatment programs are used to minimize deposits, but evaporators can become fouled over time and cleaning is in order. Options for cleaning these systems are chemical in-situ programs or mechanical cleaning.
As a result of significant deposit formation that can occur in unit operations such as evaporators, opportunities exist to improve system operations by using an effective chemical cleaning program. One option to deal with declining performance of Mechanical Vapor Compression (MVC) evaporators or evaporators in general due to deposits is to implement a chemical wash. Chemical washes may not always be completely effective for dissolving deposits. Some types of cleaning chemistries can be hazardous to both equipment and personnel. If a chemical wash does not effectively dissolve tenacious deposits, then mechanical cleaning may need to be performed. Mechanical cleaning is very time consuming, expensive (e.g., for waste removal/labor costs), and can result in significant lost production. Thus, there is a continuing need for a new chemistry to remove and/or limit deposit formation.